The air quality in unoccupied spaces such as homes, office buildings, and hotel rooms can become problematic if not regulated properly. In hot and humid climates such as Florida, for example, mold and mildew buildup can begin to occur within only a few days, particularly when the interior environment is within the psychrometric range above 72° F. and 60% relative humidity where mold spore growth is generally increased. In such climates, dehumidification is often required in order to maintain adequate indoor air quality (IAQ) levels within the interior space while the occupants are away for extended periods of time. During these relatively long periods of time, however, the occupants will often desire to conserve energy by setting the temperature at a higher level in order to reduce air-conditioner usage. A tradeoff thus exists between energy savings and sufficient humidity control.
The prevention of mold and mildew buildup in unoccupied spaces is typically accomplished using a thermostat, sometimes in series or parallel with a humidistat.
Configuration of the humidistat to work in conjunction with the thermostat is often difficult since the user must make the correct settings on both the thermostat and humidistat before leaving. Since such configuration requires a specific change in setpoint and is rarely done (e.g., once a year), the steps needed to configure both the thermostat and humidistat are often difficult to remember. If the user sets the controllers incorrectly, the result can be either insufficient humidity control due to a lack of proper dehumidification, or an excessive energy bill resulting from the air-conditioner running more than is required. To assist in proper configuration, therefore, the installer of the HVAC system will sometimes paste a long list of instructions on the wall instructing the occupants how to properly set the fan switch, the system switch, the temperature setpoint, the humidity setpoint, as well as other settings while they are away.
In those cases where the HVAC system is not equipped with a separate dehumidifier, the air-conditioner can be used in lieu of the dehumidifier to regulate the humidity levels within the space. When operated as a dehumidifier, air flowing past the air-conditioning coils results in condensation on the coils, which removes water from the air and reduces the humidity levels within the space. Efforts to lower the inside temperature to reduce humidity levels within the space can be counterproductive, however, if the inside dewpoint temperature is greater than the room temperature within the interior space. If, for example, the inside dewpoint temperature within the space is 72° F. whereas the indoor temperature is 70° F., operation of the air conditioning unit may actually cause greater moisture to buildup within the space, increasing mold and mildew growth and decreasing the indoor air quality. This may occur, for example, when the temperature sensed at the thermostat is higher than that at other locations within the interior space such as the outlet ducts to the HVAC system, causing moisture to buildup on the walls adjacent to the ducts. In addition, if the cooling provided by the air conditioner exceeds the rate of dehumidification as is common in many oversized air conditioner systems, the rapid drop in temperature may cause the system to reach the dewpoint temperature before fully satisfying the humidity requirement.